As wireless biomedical implant devices advance to smaller sizes with higher processing power, the issue of power supply becomes a critical design hurdle. Designers for biomedical devices have turned their attention to sensors that are powered by RF energy that is implanted on or within the skin. The most popular power transfer technique is inductive coupling (near-field) because attenuation in tissue is reduced in comparison to RF (far-field) traveling waves and antenna efficiency is independent of wavelength. Unfortunately, as device (antenna) size decreases power collected by the device falls off in proportion to the mutual inductance squared or R4 where R is the radius of the antenna coil. For this reason it is important that the low RF energy levels collected by the antenna are efficiently converted to DC power to operate the implant.
Similarly, efficient energy conversion is important to RFID tags. A highly efficient RFID tag can be powered at a further distance from a reader, for example. Alternatively, a highly efficient RFID tag can be more readily powered by a reader when the tag is embedded in an article or medium that attenuates or absorbs RF energy.
The voltage rectifier is a critical element that affects efficiency of power conversion from AC RF energy to DC energy required for a device such as a medical implant or an RFID tag. Conventional rectifies used in wirelessly powered devices such as UHF RFIDs, micro-sensors and biomedical implants are unfortunately extremely inefficient at low input levels. The inefficiency arises from the threshold voltage (Vth) of devices used within the rectifier, which are generally standard CMOS transistors. If the peak-to-peak RF input voltage swing is below the Vth of the devices used, the rectifier will never turn on and no DC output will be produced. This region is known as the “dead zone” and generally leads to reduced read ranges for wireless devices. See, e.g., S. Mandal and R. Sarpeshkar, “Low-Power CMOS Rectifier Design for RFID Applications,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 54, no. 6. In the case of biomedical implants, by setting the minimum power required for rectifier function, the dead zone will limit the minimum achievable implant size. Low threshold (Vth typically ˜|0.4V|) Schottky diodes have been used to reduce the dead zone but the threshold of a Schottky diode still presents a significant dead zone due to a threshold that does not approach zero, as reported in U. Karthaus and M. Fisher, “Fully integrated passive UHF RFID transponder IC with 16.7-μW minimum RF input power,” IEEE J. Solid-State Circuits, vol 38, no. 10 pp. 1602-1608, October 2003.
Other efforts have coupled with CMOS devices with Vth-cancellation techniques to improve low input performance. For example, T. Umeda, H. Yoshida, S. Sekine, Y. Fujita, T. Suzuki, and S. Otaka, “A 950-MHz rectifier circuit for sensor network tags with 10-m distance,” IEEE J. Solid-State Circuits, vol. 41, no. 1, pp. 35-41, January 2006, threshold is cancelled through the use of a static DC voltage applied to the devices. This method has the disadvantage of requiring an alternate power source to supply this canceling voltage, making a passive design impossible. Passive cancellation designs are reported in other publications that utilize DC power generated by the rectifier itself to help overcome the threshold of the devices used. While this approach can provide high efficiencies at low input power levels, it still requires that DC power is generated by the rectifier before any cancellation can take place and therefore fails to address turn-on voltage. See, e.g., S. Guo; H. Lee; “An Efficiency-Enhanced CMOS Rectifier With Unbalanced-Biased Comparators for Transcutaneous-Powered is High-Current Implants,” IEEE J. Solid-State Circuits, vol. 44, no. 6, pp. 1796-1804, June 2009; C.-L. Chen, K.-H. Chen, S.-I. Liu, “Efficiency-enhanced CMOS rectifier for wireless telemetry,” Electronics Lett., vol. 43, no. 18, pp. 976-978, August 2007; C.-S. A. Gong, K.-W. Yao, J.-Y. Hong, K.-Y. Lin, M.-T. Shiue, “Efficient CMOS rectifier for inductively power-harvested implants,” Electron Devices and Solid-State Circuits, 2008, 8-10 Dec. 2008, pp. 1-4; K. Kotani, T. Ito, “High efficiency CMOS rectifier circuit with self-Vth-cancellation and power regulation functions for UHF RFIDs,” Solid-State Circuits Conf., San Francisco, Calif., 12-14 Nov. 2007, pp. 119-122; K. Kotani, A. Sasaki, and T. Ito, “High-Efficiency Differential-Drive CMOS Rectifier for UHF RFIDs,” IEEE J. Solid-State Circuits, vol. 44, no. 11, pp. 3011-3018, November 2009; S. Mandal, R. Sarpeshkar, “Low-Power CMOS Rectifier Design for RFID Applications,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 54, no. 6, T. Umeda, H. Yoshida, S. Sekine, Y. Fujita, T. Suzuki, and S. Otaka, “A 950-MHz rectifier circuit for sensor network tags with 10-m distance,” IEEE J. Solid-State Circuits, vol. 41, no. 1, pp. 35-41, January 2006. For the rectifiers in this style of approach, the peak-to-peak voltage of the incoming signal must reach a magnitude greater than Vth for the rectifiers to initially turn on when no DC voltage is present at the rectifier's output.
J. Yi, W.-H. Ki; C.-Y. Tsui, “Analysis and Design Strategy of UHF Micro-Power CMOS Rectifiers for Micro-Sensor and RFID Applications,” IEEE Trans. Circuits Syst. 1, Reg. Papers, vol. 54, no. 1, pp. 153-166, January 2007, discloses a charge pump rectifier design that uses advanced process CMOS low or near zero threshold transistors. The charge pump design was reported to achieve a rectifier efficiency of 26.5% at an input power of −11.12 dBm for UHF micro sensor applications. A limitation of the charge pump diode design is that the rectifier's loss over the RF cycle is dependent upon the load.
CMOS coupled designs have advantages over the charge pump diode designs, but artisans have avoided low and near zero threshold transistors because of losses caused by device reverse conduction around zero crossings of the input RF signal. FIG. 1 illustrates a cross-coupled bridge rectifier that uses optimized low threshold transistors from “S. Mandal, R. Sarpeshkar, ”Low-Power CMOS Rectifier Design for RFID Applications,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 54, no. 6, As the output voltage increases, a DC offset voltage builds up at the devices gates (between VinRF and ground or VoutDC), this causes the devices to remain on during zero crossings in the RF input cycle leading to reverse conduction and power loss. The threshold voltage of the devices used is set to an optimal value where reverse conduction is minimized and switch on resistance is minimized at the target output voltage. This leads to peak efficiency at a single target output value.